Problems with Samsung 740N monitors

[Behemot]

Re: Problems with Samsung 740N monitors

Guess its 7.5 V either single with two pads connected to one end, or two in parallel. Should not matter anyway.

[budm]

Re: Problems with Samsung 740N monitors

Can we see the straight shot pictures of the top and the bottom side of the board?

[jondoe]

Re: Problems with Samsung 740N monitors

I've lifted a number of the large resistors to check them out of circuit, and most of the transistors/large mosfets, they all appear to be OK.

Pictures attached

Edit: I borrowed some cheap calipers, the chip is approx 2.9mm wide at it's longest point, so I suspect this is a SOT23 package.

[jondoe]

Re: Problems with Samsung 740N monitors

In the meantime I've ordered two IRF7319PBF and a BZX84C7V5LT1G. I'm fairly sure those are suitable replacement parts, I'll find out next week!

[budm]

Re: Problems with Samsung 740N monitors

It looks like R217 is connected in parallel (need verification) with ZD201. Did you test the ZD201 off the board?
The Cathode (PIN 3) of ZD201 is connected directly to the B+ for running the inverter circuit.

[jondoe]

Re: Problems with Samsung 740N monitors

I didn't test it off the board, but I did compare readings with a working, identical board and the two were wildly different, I guess that's not a great way to test it. I'll remove it and test regardless at some point tomorrow.

Thanks!

[budm]

Re: Problems with Samsung 740N monitors

The reading error may be due to the bad 8-PIN SMD Power MOSFETs.

[jondoe]

Re: Problems with Samsung 740N monitors

A very valid point, that did cross my mind. I could always change the mosfets first before the zener and test

[jondoe]

Re: Problems with Samsung 740N monitors

Well, that was a little more difficult to remove than I had hoped, she was glued and didn't want play.

I've tested it off the board with my DMM

  |
[ZD201]
|   |
0.7   OL

One leg doesn't appear to work at all, which according to the datasheet I think is correct (at least it is with the replacement ordered). So I assume this was working fine.

I'll replace it with the new one as the old one looks a little worse for being pulled out with my ham fists

[Behemot]

Re: Problems with Samsung 740N monitors

Yeah they usually glue them to hold in place during wave soldering.

Nice work with the magnifying glass BTW, I am surprised you managed to focus it

[jondoe]

Re: Problems with Samsung 740N monitors

I used a dirt cheap plastic jewelers loupe and my camera phone for the picture, they come out surprisingly well

[jondoe]

Re: Problems with Samsung 740N monitors

That was a little harder than I imagined, but the MOSFETs are in place, the new zener and a fresh fuse. The heat sink on the back of the MOSFETs common ground makes soldering a pain in the butt, removal before hand might have been easier.

I destroyed one pad while removing one of them, but managed to bend the leg a little and scrape some trace so it could take a tiny bit of solder make the connection.

I won't post a picture of the work, I'm too embarrassed

It fired up first time, back lights working, etc, so I'm leaving it on the bench to see if it pops a fuse again.

[jondoe]

Re: Problems with Samsung 740N monitors

So far, so good. I left it turned on all day yesterday and last night and she's still running. Thanks to everyone who contributed in this thread, past and present

[Venomus28]

Re: Problems with Samsung 740N monitors

Did you changed the 2 mosfets too ?

[jondoe]

Re: Problems with Samsung 740N monitors

In all, I changed 3 of the large capacitors, 1 pico fuse, 2 x MOSFETs (replaced with IRF7319PBF) and a zener diode (BZX84C7V5LT1G) although I didn't need to change the Zener, I just destroyed when removing it to test.

It's still working fine today, i turned it off over night to cool then powered it back up this morning.

[Venomus28]

Re: Problems with Samsung 740N monitors

Was it hard to solder the new mosfets ?
I have had 2 ,Samsung 940T and on the 1st one (im using now) i changed 3 caps and one pico fuse
and at the other one i have changed 3 caps and the pico fuse and i am waiting for the mosfets to get delivered so i can change them too

[jondoe]

Re: Problems with Samsung 740N monitors

How easy it is will depend on your own experience and the tools you have. Removing them was probably harder than replacing them, the glue holding them in place didn't want to let go. The heat sink also made it harder work than it needed to be, removal of that before hand might have made it easier.

I used a little solder wick to clean the pads up before placing the new ones on the board, they aren't perfectly straight, but they are all working. I do have a cheap hot air station, it's quality is questionable so I managed without it!

[Venomus28]

Re: Problems with Samsung 740N monitors

Yes i know, can you put up a pic ?

[jondoe]

Re: Problems with Samsung 740N monitors

The monitor is reassembled now being tested, so I can't take a picture, sorry.

[Venomus28]

Re: Problems with Samsung 740N monitors

After changing both mosfets the monitor turned on and worked right for about 10 mins then it the back light turned off again
What could it be ?